Unlike a conventional microscope, the STM can detect electrons constrained in a sample. In recent years, the STM has received a great deal of attention as a typical surface observation apparatus capable of observing atomic alignment in a real space. The principle of operation of such an STM will be described below.
A probe having a sharp tip comes close to a sample surface by an z-direction actuator such that electron clouds smeared out from the sample surface slightly overlap each other, and a voltage (tunnel voltage) is applied between the probe and the sample to cause a tunnel current to flow from the probe to the sample. The z-direction actuator is servo-controlled to keep this tunnel current constant. At the same time, the probe and the sample are relatively moved in a surface direction by an xy-direction actuator to perform two-dimensional scanning. At this time, a servo voltage applied to the z-direction actuator which servo-controls the probe is read, and the read voltage is displayed as an image, thereby observing the surface of the sample. That is, the probe scans the sample surface. When a scanning position reaches a step on the sample surface, a tunnel current is increased. The probe is separated from the sample by the z-direction actuator until the tunnel current reaches a constant value (initial value). Since this probe movement corresponds to the step on the surface, this scanning operation is repeated to read servo voltages, thereby obtaining a surface image of the sample.
The tunnel current J.sub.T is represented by the following relation: EQU JT.varies.exp(-A.multidot..phi..sup.1/2 .multidot.S)
where
A: a constant PA1 .phi.: an average of work functions of the probe and the sample PA1 S: a distance between the probe and the sample
The tunnel current JT therefore changes in accordance with a change in distance S with high response, and a resolution of an atomic scale can be obtained.
As described above, the STM can obtain a surface image of a substance with a high resolution. Unlike a reciprocal lattice space image obtained by a method such as electron beam diffraction or ion scattering, the STM has a characteristic feature capable of observing atomic alignment in a real space. In addition, a voltage applied between the probe and the sample has a value smaller than the work function of the sample. Since the tunnel current is detected on the nA order, power consumption is low, and the damage to the sample is little.
Although a conventional STM can obtain a surface image having a very high resolution in the real space, an observation portion is unclear or the STM is not suitable for observation for a specific portion within a narrow range because the observation portion on the sample surface is observed with eyes and the above observation operation is performed. In addition, an STM image can't be compared with a conventional image obtained by other microscopes (e.g., an optical microscope and an electron microscope), and an STM observation region (STM field) does not necessarily coincide with the conventional observation field.
The present invention therefore has been made in consideration of problems of the above prior art, and has as its object to provide a scanning tunnel microscope capable of allowing an STM image to overlap a conventional image and observing and measuring the STM image.